The present invention relates to a negative pressure booster for use with a brake master cylinder of an automobile.
Conventionally, negative pressure boosters of this type include a housing and a power piston which divides the interior of the housing into a suction pressure chamber and an atmospheric pressure chamber. The suction pressure chamber is connected with the engine intake passage so that the suction pressure produced in the engine intake passage is introduced into the suction pressure chamber. The power piston has a communication passage which connects the suction pressure chamber with the atmospheric pressure chamber through valve means. The atmospheric pressure chamber is also provided with second valve means which is normally closed but functions to selectively open the atmospheric pressure chamber to the atmosphere. When the brake is released, the first valve means is opened to connect the two chambers with each other and the second valve means closed to disconnect the atmospheric pressure chamber from the atmosphere, so that the pressures in the two chambers are balanced and the power piston is maintained in the retracted position. When the brake pedal is depressed, however, the first valve means is closed and the second valve means is opened so that the atmospheric pressure is introduced into the atmospheric pressure chamber. Thus the power piston is forced under the pressure difference between the two chambers to move the output rod connected therewith.
As disclosed in the U.S. Pat. No. 4,282,799 issued on Aug. 11, 1981 to H. Takeuchi, the power piston in the conventional negative pressure booster comprises a diaphragm having a periphery secured to the housing and a central hub to which the output rod is connected. The central hub is made of a plastic material and formed with a central hole of a stepped configuration. The central hole includes a large diameter hole section facing the suction pressure chamber and a small diameter hole section connected to the large diameter section through a stepped shoulder portion. The output rod has an output piston which is formed at one end thereof and axially slidably received in the large diameter hole section in the central hub. In the bottom portion of the large diameter hole section, there is a resilient piston which is usually made of a rubber and a reaction piston is disposed in the small diameter hole section for the purpose as described in the aforementioned U.S. patent.
In this type of booster, the central hub is associated with the brake actuating push rod through a ball joint so that the central hub can incline with respect to the axis of the push rod. However, since the central hub is connected with the output rod through the slidable engagement between the output piston on the output rod and the hole in the central hub, the output rod is always maintained in the axially aligned position with respect to the central hub of the power piston. Thus, any tendency of inclining the power piston is resisted by the output piston which applies a radial reaction force to the wall of the large diameter hole section. Further, any axial pressure which may be applied to the resilient piston in brake operation tends to make it radially expand so that the wall of the large diameter hole section is additionally applied with a radially directed pressure. Due to these radially directed forces, the conventional structures have problems in that the central hub may become cracked particularly at the corner of the stepped shoulder portion.
A further problem encountered in the conventional negative pressure booster is that it is bulky as compared with other automobile components. In order to provide a required amount of output, the booster must necessarily have a certain amount of pressure acting area as well as sufficient operating stroke movement of the power piston. It should however be noted that in the conventional structures there is provided an unnecessary clearance between the power piston and the housing in the extended position. Further, the central hub of the power piston must have a sufficient axial length so that the parts of the valve mechanism are housed therein and due to the axial length of the central hub it has been difficult to make the booster compact.